ORIGINAL PAPER

Photosynthetic responses in Phaeocystis antarcticatowards varying light and iron conditions

M. A. van Leeuwe Æ J. Stefels

Received: 23 March 2006 / Accepted: 10 August 2006 / Published online: 23 March 2007� Springer Science+Business Media B.V. 2007

Abstract The effects of iron limitation on

photoacclimation to a dynamic light regime were

studied in Phaeocystis antarctica. Batch cultures

were grown under a sinusoidal light regime,

mimicking vertical mixing, under both iron-suffi-

cient and -limiting conditions. Iron-replete cells

responded to changes in light intensity by rapid

xanthophyll cycling. Maximum irradiance coin-

cided with maximum ratios of diatoxanthin/dia-

dinoxanthin (dt/dd). The maximum quantum

yield of photosynthesis (Fv/Fm) was negatively

related to both irradiance and dt/dd. Full recovery

of Fv/Fm by the end of the light period suggested

successful photoacclimation. Iron-limited cells

displayed characteristics of high light acclimation.

The ratio of xanthophyll pigments to chlorophyll

a was three times higher compared to iron-replete

cells. Down-regulation of photosynthetic activity

was moderated. It is argued that under iron

limitation cells maintain a permanent state of

high energy quenching to avoid photoinhibition

during exposure to high irradiance. Iron-limited

cells could maintain a high growth potential due

to an increased absorption capacity as recorded

by in vivo absorption, which balanced a decrease

in Fv/Fm. The increase in the chlorophyll a-

specific absorption cross section was related to

an increase in carotenoid pigments and a reduc-

tion in the package effect. These experiments

show that P. antarctica can acclimate successfully

to conditions as they prevail in the Antarctic

ocean, which may explain the success of this

species.

Keywords Fluorescence � Iron � Light �Phaeocystis � Pigments � Xanthophyll cycling

Introduction

In the Southern Ocean, the factors light and iron

exert synergistic growth control on microalgae

(De Baar et al. 2005). In vast areas, iron concen-

trations are often too low to support growth

(Martin et al. 1990; van Leeuwe et al. 1997). In

addition, frequent wave action induces deep

vertical mixing of the water column. Conse-

quently, its planktonic inhabitants are subject to

a highly dynamic light regime (Mitchell and

Brody 1991). Under these conditions, the ability

to photoacclimate determines the growth success

of algae. Since iron exerts control on various

photosynthetic processes, its low ambient

M. A. van LeeuweDepartment of Marine Biology, Biological Centre,University of Groningen, Haren, The Netherlands

J. Stefels (&)Laboratory for Plant Physiology, Biological Centre,University of Groningen, P. O. Box 14,Haren 9750 AA, The Netherlandse-mail: j.stefels@rug.nl

123

Biogeochemistry (2007) 83:61–70

DOI 10.1007/s10533-007-9083-5

concentration may impede photoacclimation

(Greene et al. 1991; van Leeuwe and De Baar

2000). Yet, the impact of iron limitation on

photoacclimation has so far received little atten-

tion.

Photoacclimation to a dynamic light regime

involves optimisation of the photosystems in such

a way that energy generation and utilisation are in

balance (Kana et al. 1997). Conditions of rapid

vertical mixing require that cells can acclimate

directly to changes in light intensity that range

from almost darkness to excessively high irradi-

ance levels. Dim light conditions deep in the

water column necessitate an extensive light-har-

vesting complex, whereas high irradiance levels at

the surface require small complexes that are

resistant to photodamage. When more energy is

generated than can be utilised for growth, photo-

damage can occur (Demmig-Adams and Adams

1996). Under variable light conditions, periods

between low- and high-light conditions are usu-

ally too short for acclimation by de novo synthe-

sis of light-harvesting or photoprotective

pigment-protein complexes, respectively. More-

over, de novo synthesis is a costly process. Cells

that are subjected to variable light conditions will

therefore benefit from flexible photosystems (see

Muller et al. 2001 for review). Processes involved

in photoacclimation on short time scales (min-

utes) are energy-dependent quenching (qE) and

state-transition quenching (qT). These processes

vary with species (e.g. van Leeuwe et al. 2005).

Green flagellates possess adaptable photosystems

that support the rapid down-regulation of photo-

synthetic activity by xanthophyll pigment cycling

and state transitions. In these organisms, alterna-

tive quenching processes that involve mobility of

the photosynthetic membranes (Anderson et al.

1995) may also play a role, but are not well

described. In contrast, rapid photoacclimation in

diatoms is limited to xanthophyll cycling. This

appears to be related to a more-homogeneous,

and therefore less-flexible, organisation of the

thylakoid membranes of diatoms (Larkum et al.

2003).

The make-up of the photosynthetic apparatus

and consequently to the capacity of photoaccli-

mation depends on the availability of iron. Pre-

viously it has been found that iron-limited cells

contain less chlorophyll a, but relatively more

photoprotective pigments (Geider et al. 1993; van

Leeuwe and Stefels 1998; van Leeuwe and De

Baar 2000). In addition, various protein-pigment

complexes that constitute the photosystems re-

quire iron for synthesis (cytochromes and FeS

proteins). Under iron limitation, impairment of

those protein complexes results in a decrease in

the efficiency of electron transport (Greene et al.

1991). Experiments that were carried out under

low, stable light conditions showed that while iron

limitation resulted in a reduction in photosyn-

thetic efficiency, the absorption efficiency had

increased due to a reduction in the package effect

(Greene et al. 1991; van Leeuwe and De Baar

2000). Accordingly, iron-limited cells were accli-

mated to low irradiance to a large extent. How-

ever, nothing is known about the effects of iron

limitation on the photosynthetic response towards

variable light conditions. A disorder in the archi-

tecture of the photosynthetic membranes may

have negative consequences for the flexibility of

the photosystems through constraining effects on

the coupling and decoupling of pigment-protein

complexes that are involved in xanthophyll

cycling (Muller et al. 2001). Even so, alternative

quenching processes that depend on conforma-

tional changes of membrane proteins are likely to

be disturbed.

So far, few studies addressed photoacclimation

to rapidly changing light conditions. The data that

are available show that diatoms acclimate to the

average irradiance levels they encounter. Under

variable light conditions, this strategy has nega-

tive consequences for their growth success as it

makes the algae more susceptible to photoinhibi-

tion when exposed to high irradiance levels

(Flameling and Kromkamp 1995; Flameling

1998, van Leeuwe et al 2005). No studies are

currently available on the effects of iron limita-

tion on photoacclimation towards a variable light

regime. Here, we present the first data on

photosynthetic responses by the marine microalga

Phaeocystis antarctica towards varying conditions

of light and iron availability. P. antarctica is

abundant in the Southern Ocean, and it plays a

prominent role in various biogeochemical cycles

(Smith et al. 1998; Arrigo et al. 1999; Vaillan-

court et al. 2003). The alga may form massive

62 Biogeochemistry (2007) 83:61–70

123

blooms that can potentially sequester CO2 from

the atmosphere. In addition, P. antarctica is

known for its production of dimethylsulphide

(DMS), a semivolatile sulphur compound that

once it has escaped into the atmosphere induces

cloud formation thus affecting the global climate

system (Crocker et al. 1995; Stefels and van

Leeuwe 1998). The success of Phaeocystis as a

cosmopolitan species is still not well understood.

Previously, rapid photophysiological acclimation

was found in P. antarctica (Moisan et al. 1998; van

Leeuwe and Stefels 1998; Moisan and Mitchell

1999). These studies were performed under stable

light conditions and they only partly explain its

success in the open ocean areas that are charac-

terised by highly dynamic light conditions (Mitch-

ell and Brody 1991). Here we will show that P.

antarctica can be so abundant in the Southern

Ocean because of its high capacity to acclimate its

photosystems to ambient irradiance levels, even

under conditions of iron limitation. Photosyn-

thetic efficiency was recorded by means of dual-

modulation fluorescence analysis. Analysis of the

pigment composition and recording of in vivo

absorption spectra were used to determine

responses of the antennae complex.

Methods

Culture conditions

Phaeocystis antarctica (CCMP 1871; non-axenic)

was cultured under dynamic light conditions

simulating vertical mixing through the water

column. The light regime was enforced by a

Venetian Blind system that was mounted between

the cultures and the light source (Philips MHN-

TD). The system consisted of horizontal slats the

position of which could be altered by a step

motor. Two independent units were used to

enforce variable light conditions; the diurnal

intensity of solar irradiance was simulated by a

sinusoidal light course. Vertical mixing was sim-

ulated by a second oscillation that was superim-

posed on the solar pattern (see Fig. 1). The

maximum intensity of irradiance was 400 lmol

photons m–2 s–1. The light inside the culture flasks

was measured with a spherical sensor (Biosperical

Instruments Inc. QSL-100). The cultures were

grown in 2L polycarbonate erlenmeyer flasks at

4�C and kept in suspension by daily shaking.

Medium was prepared as described by Morel

et al. (1979). Chelexed, nutrient-poor seawater

originating from the Atlantic sector of the South-

ern Ocean was enriched with 140 lM NO3,

7.5 lM PO4 and 60 lM Si. Solutions of FeCl3and ethylenediaminetetraacetic acid (EDTA)

were added separately at final concentrations of

10–6 M EDTA for all cultures and 10–6 M iron for

iron-rich cultures. Initial iron concentrations

(measured by solvent extraction followed by

atomic absorption spectrophotometry; Nolting

and de Jong 1994) were 15 nM. Before starting

the experiments, cultures were stressed by iron

deficiency and adapted to light conditions for

several weeks. During that period, the main

growth form of the cells had gradually changed:

the iron-limited cultures consisted mainly of

single cells; the iron-replete cultures consisted

mainly of colonies.

Photosynthetic parameters

Chlorophyll a fluorescence was measured with a

dual-modulated fluorometer (Photosystem Instru-

ments, Trtı́lek et al. 1997). Minimum (Fo) and

maximum fluorescence (Fm) was recorded after

dark adaptation (5 min at 4�C, sufficient to attain

stabilization of the fluorescence signal for all light

regimes). The maximum quantum yield of pho-

tosynthesis Fv/Fm was calculated as (Fm–Fo)/Fm

(Krause and Weis 1991).

Samples for pigment analysis were filtered

gently (<15 KPa) over Whatman GF/F filters

and subsequently snap-frozen in liquid nitrogen

and stored at –80�C until analysis. Before extrac-

tion in 90% acetone filters were freeze-dried

during 48 h. Pigments were analysed by high-

performance liquid chromatography on a Waters

system equipped with a photodiode array (van

Leeuwe et al. 2006). A Waters DeltaPak re-

versed-phase column (C18, fully end-capped)

was used. Pigment standards were obtained from

the DHI Water Quality Institute (Horsholm,

Denmark).

Biogeochemistry (2007) 83:61–70 63

123

In vivo light absorption spectra (400–700 nm)

were recorded with a Varian-Cary spectropho-

tometer equipped with an integrating sphere.

Samples were taken to an approximate optical

density of 0.1. Data were corrected for scattering

by subtracting a baseline value as measured at

725 nm. The spectrally averaged, chlorophyll a

specific absorption cross section (�aph, m2 mg Chl

a–1) was calculated as follows:

�aph ¼

R700

400

achlðkÞ �QðkÞdðkÞ

R700

400

QðkÞdðkÞ

where achl(k) is the wavelength specific absorp-

tion (m2 mg Chl a–1), Q(k) is spectral irradiance

of the light source (lmol photons m–2 s–1) and

d(k) is the wavelength interval of the spectral

band (2 nm).

Sampling strategy and statistical analyses

All experiments were performed in duplicate.

Samples for photosynthetic parameters were

taken over the course of one day only (13 data

points). A day was chosen during which cells were

still growing exponentially while cell densities

were high enough to allow reliable in vivo fluo-

rescence measurements. Growth rates were estab-

lished by cell counts in the days preceding the

24 h experiments. Cells were counted with an

inverse microscope after settling for 24 h in a

counting chamber. Effects of iron limitation on

photosynthetic parameters (Table 2) were tested

for significance by use of a two-factor analyses of

variance. The daily dynamics of the photosyn-

thetic parameters (Figs. 1–3) were tested for

significance by multiple regression analyses. Dif-

ferences were considered significant at P < 0.05.

Results

The main pigments of Phaeocystis antarctica were

chlorophyll a, chlorophyll c1 + 2, fucoxanthins,

diadinoxanthin and diatoxanthin. The ratio of

the photoprotective xanthophyll pigments diadi-

no- and diatoxanthin to chlorophyll a was 0.229 in

iron-limited cells and only 0.077 in iron-replete

cells (Table 1). The ratio of 19¢-hexanoyloxyfuco-

xanthin to chlorophyll a was also higher under

iron limitation: 0.734 versus 0.344 for iron-replete

cells. Iron limitation resulted in 10% growth

reduction (Table 2). The reduction in growth was

associated to a 20% reduction in the maximum

quantum yield of photosynthesis (Fv/Fm) and

40

60

80

100

120

140

–

100

200

300

400

500

40

60

80

100

120

140

7:00 10:00 13:00 16:00 19:00 22:00 1:00

Time (h)

–

100

200

300

400

500

40

60

80

100

120

140

Fv/

Fm

(%

)–

100

200

300

400

500

Irradiance (µmol m

–2 s–1)

Fo

(%

)F

m (

%)

Irradiance (µmol m

–2 s–1)

Irradiance (µmol m

–2 s–1)

Fig. 1 Fluorescence characteristics of photosynthesisdetermined over the day for iron-limited (open symbols)and iron-replete cells (closed symbols), plotted alongsideirradiance (photons m–2 s–1; dashed line). Fluorescencedynamics are expressed in percentages, taking themaximum morning values as reference (i.e. 100%;n = 3). (A) Maximum quantum yield of fluorescence(Fv/Fm). (B) Minimum fluorescence (Fo). (C) Maximumfluorescence (Fm)

64 Biogeochemistry (2007) 83:61–70

123

almost 50% decrease in cellular chlorophyll a

content (Table 2, Fig. 4).

The Fv/Fm of 0.5 recorded for iron-replete cells

was close to the maximum values measured in our

lab, though they are lower than the values reported

for natural populations that were dominated by

Phaeocystis antarctica (Vaillancourt et al. 2003).

Whereas our cultures were in good condition, the

experimental conditions provided were certainly

not the most optimal for growth P. antarctica.

Maximum values for Fv/Fm were in fact achieved at

more stable light conditions (pers. obs.).

The maximum quantum yield of photosynthesis

(Fv/Fm) varied with daily dynamics in light inten-

sity in both the iron-limited and iron-replete cells

(Fig. 1A). Maximum values were recorded in the

morning, followed by decreasing values towards

the middle of the light period. In replete cells, the

midday depression was almost 50% of the morning

values; Fv/Fm decreased by about 30% under iron

limitation. The Fv/Fm recovered towards the end of

the light period (significant relationship between

irradiance and Fv/Fm; ANOVA, P < 0.05).

Depressions in irradiance coincided with moderate

increases in the fluorescence parameters. Mini-

mum (Fo) and maximum (Fm) fluorescence values

varied along with Fv/Fm (Fig. 1B, C). The dynamics

were most pronounced for the maximum fluores-

cence signal in iron-replete cells.

In concordance with the diel dynamics in

Fv/Fm, active xanthophyll cycling was observed

over the day (Fig. 2A, B). The ratio of diatoxan-

thin (dt) to diadinoxanthin (dd) was closely linked

to irradiance, with maximum conversion of dd

into dt recorded during maximum irradiance.

Whereas the total pool of dd + dt was highest in

iron-limited cells (Table 1), the dt/dd ratio was

significantly higher in iron-replete cells: 0.9 versus

0.15 for Fe-limited cells (Fig. 2).

In iron-deplete cells, the pool of fucoxanthins

was not stable over the day. The ratio of

19¢-hexanoyloxyfucoxanthin to fucoxanthin

Table 1 The pigment composition of Phaeocystis antarctica for iron-limited and iron-replete cells, expressed as ratios tochlorophyll a (n = 3) (average values ± s.d.)

Ratio to Chl a Fe-limited ± s.d. Fe-replete ± s.d.

Chlorophyll c3 0.116 0.012 0.091 0.009Chlorophyll c2 0.197 0.017 0.202 0.01319¢-Butanoyloxyfucoxanthin 0.013 0.003 0.009 0.003Fucoxanthin 0.011 0.004 0.017 0.00919¢-Hexanoyloxyfucoxanthin 0.734 0.021 0.344 0.120Diadinoxanthin 0.214 0.037 0.057 0.048Diatoxanthin 0.017 0.012 0.025 0.024Zeaxanthin 0.026 0.002 0.009 0.003Beta-carotene 0.016 0.004 0.014 0.007Total fucos* 0.760 0.025 0.381 0.138Diadino-plus diatoxanthin 0.229 0.044 0.077 0.065

* 19¢-Butanoyloxyfucoxanthin + Fucoxanthin + 19¢- Hexanoyloxyfucoxanthin

Table 2 Photosynthetic parameters of Phaeocystis antarctica for iron-limited and iron-replete cells (n = 3) (average values± s.d.)

l (d–1) ± s.d. Fv/Fma ± s.d. Chlorophyll

a (pg cell–1)± s.d. �aph

(m2 mg Chl–1)± s.d. POCb

(pg cell–1)rgc c

Fe-limited 0.49 0.02 0.40 0.04 0.08 0.02 0.057 0.023 14 0.13Fe-replete 0.55 0.07 0.50 0.02 0.15 0.03 0.030 0.022 19 0.12

Differences between the treatments were significant [analysis of variations (ANOVA) P < 0.05]) for all parametersa Morning valuesb Data from Stefels and van Leeuwe (1998)c Relative growth capacity, see discussion for details

Biogeochemistry (2007) 83:61–70 65

123

(HEXA/FUCO) decreased towards the end of

the light period (Fig. 3A; P < 0.05), which was

associated with a 50% increase in FUCO/Chl a

(Fig. 3B), whereas HEXA/Chl a remained invari-

able at a high level (Fig. 3C). In iron-replete cells,

no significant daily dynamics in HEXA and

FUCO were observed (Fig. 3D–F).

In both treatments, the in vivo light absorption

characteristics were invariable over the day (data

not shown). Maximum values in the chlorophyll-

specific absorption coefficient were observed near

675 nm (Chl a), 470 nm (carotenoids) and 430 nm

(Chl a) (Fig. 4). The absorption cross section was

0.057 m2 mg Chl a–1 under iron limitation and

0

0.005

0.01

0.015

0.02

0.025

fuco

/ ch

l.a

0

0.01

0.02

0.03

0.04

0.050.05

–

100

200

300

400

500

0

0.2

0.4

0.6

0.8

1

7:00 10:00 13:00 16:00 19:00 22:00 1:00

Time (h)

hex

al /

chl.a

0

0.2

0.4

0.6

0.8

1

7:00 10:00 13:00 16:00 19:00 22:00 1:00

Time (h)

–

100

200

300

400

500

Fe–

020406080

100120140

hex

a / f

uco

Fe+

0

10

20

30

40

50

–

100

200

300

400

500A

B

D

E

C F

Irradiance(µm

ol m–2 s

–1)Irradiance

(µmol m

–2 s–1)

Irradiance(µm

ol m–2 s

–1)

Fig. 3 Daily dynamics of the fucoxanthin pigments plottedalongside irradiance (photons m–2 s–1; solid line) in iron-limited (Fe–) and iron-replete (Fe+) cells. (A, D) Ratio of

19¢-hexanoyloxyfucoxanthin to fucoxanthin (hexa/fuco).(B, E) Ratio of fucoxanthin to chlorophyll a. (C, F) Ratioof 19¢hexanoyloxyfucoxanthin to chlorophyll a

Fe–

0

0.05

0.1

0.15

0.2

7:00 10:00 13:00 16:00 19:00 22:00 1:00

Time (h)

dt/d

d

Fe+

0

0.2

0.4

0.6

0.8

1

7:00 10:00 13:00 16:00 19:00 22:00 1:00

Time (h)

–

100

200

300

400

500

Irradiance (µmol m

–2 s–1)

Fig. 2 Daily dynamics of the xanthophylls cycle (ratio of diadinoxanthin to diatoxanthin; dt:dd) plotted alongsideirradiance (photons m–2 s–1; solid line). (A) Iron-limited cells. (B) Iron-replete cells

66 Biogeochemistry (2007) 83:61–70

123

almost two times lower in iron-replete cells

(Table 2). Elevated values in the Soret region

for iron-limited versus iron-replete cells reflected

the relative increase in fucoxanthins and photo-

protective pigments.

Discussion

Cells that are exposed to varying irradiance levels

need to tune their photosynthetic apparatus in a

way that allows optimal use of irradiance during

exposure to the lower intensities and simulta-

neously find a way to avoid photoinhibition when

subjected to high irradiance levels. On short time

scales, xanthophyll pigment cycling plays an

important role in photoacclimation to high irra-

diance (Olaizola et al. 1994). De-epoxidation of

the light-harvesting pigment diadinoxanthin into

the heat dissipating diatoxanthin requires only

seconds to minutes, depending on the species.

Until now, physiological studies on xanthophyll

cycling mostly involved short-term experiments

during which abrupt light shifts were applied. A

role for xanthophyll cycling under conditions of

continuous variations in light intensity only had

been recorded in an Antarctic diatom and a green

flagellate (van Leeuwe et al. 2005). Here we show

that also in Phaeocystis antarctica, photoacclima-

tion to variable light conditions is promoted by

the xanthophyll pigments. A significant relation-

ship was recorded between the de-epoxidation

state of the xanthophyll pigments and the max-

imum quantum yield of photosynthesis (Fv/Fm).

Minimum values for Fv/Fm, coinciding with

increasing irradiance, were mirrored by enhanced

ratios of diato- to diadinoxanthin. The close

relationship between the two parameters

unequivocally establishes the role of the xantho-

phyll pigments in the down-regulation of photo-

synthetic activity. Daily dynamics in fluorescence

signals have also been observed in experiments

with cultures of P. globosa (Flameling and

Kromkamp 1995). The authors could, however,

not demonstrate a role for xanthophyll pigments.

Possibly, in those experiments the levels in

diatoxanthin were too low to determine accu-

rately. A role for xanthophyll pigments in photo-

acclimation by Phaeocystis sp. has been shown for

stable light conditions (Moisan et al. 1998; van

Leeuwe and Stefels 1998; Meyer et al. 2000). Our

study confirms these findings, and adds an active

role in the regulation of photosynthesis under

conditions of a—ecologically more rele-

vant—dynamic light climate.

Since we worked in batch cultures and iron

concentrations in the unamended media were at

nanomolar levels, initial growth rates were only

slightly reduced in the iron-limited cultures

(Table 2). The addition of EDTA did not set this

right; it appears as though Phaeocystis, amongst

other algal species, might access EDTA-com-

plexed iron (see Sedwick et al. 2006). Iron limi-

tation was achieved in the course of time, as most

clearly reflected in the photosynthetic parameters

that were recorded during the final stages of

exponential growth. A modest albeit significant

reduction in Fv/Fm was observed in iron-limited

cells. The impact of nutrient limitation on this

fluorescence parameter per se is under debate

(see Kruskopf and Flynn 2006 and references

therein). Yet, the daily dynamics provided the

more important information. Down-regulation of

Fv/Fm was suppressed under iron limitation. The

moderation in regulation was related to a weaker

response in Fm (Fig. 1). Iron limitation exerted a

minor effect on F0. These effects are most likely

related to a reduction in xanthophyll pigment

cycling. Although the patterns in de-epoxidation

state were similar for both iron conditions, the

conversion of dd into dt was relatively minor in

Phaeocystis antarctica

0.00

0.05

0.10

0.15

400 500 600 700

Wavelength (nm)

Ab

sorp

tio

n/c

hl.a

(m

2 m

g c

hl.a

–1)

Fig. 4 In vivo absorption spectra expressed per chloro-phyll a for iron-limited (open symbols) and iron-repletecells (closed symbols)

Biogeochemistry (2007) 83:61–70 67

123

iron-limited cells. The enlarged xanthophyll pool

in iron-limited cells most probably compensated

for the lower dt/dd ratio, thereby maintaining a

high level of heat dissipation throughout the day,

with minor variations over the light period. This

pattern characterises high light acclimated cells,

which confirms a high susceptibility towards

photoinhibition known for iron-limited cells

(Geider and LaRoche 1994; Morales et al. 1998;

van Oijen et al. 2004). In addition, it can be

hypothesised that the single cell shape will have

rendered iron-limited cells more sensitive to high

irradiance compared to the colonial form that

dominated under iron-replete conditions because

of an increase in specific absorption (Morel and

Bricaud 1981; Fujiki and Tagushi 2002).

Similar differences in pigment composition

between iron-limited and iron-replete cells have

previously been recorded for P. antarctica, albeit

not under variable light conditions (van Leeuwe

and Stefels 1998). When subject to a dynamic

light regime, more energy has to be invested in an

increased xanthophyll pool if the photosystems

are not flexible enough to respond to rapid

increases in irradiance level. We hypothesise that

a loss in flexibility is related to a role of iron in the

integrity and fluidity of photosynthetic mem-

branes. Photochemical quenching by xanthophyll

cycling (qE) involves conformational changes of

the photosynthetic membranes (Mohanty et al.

1995; Pascal et al. 2005). These membranes are

made up of various protein complexes in many of

which iron plays an essential role. Under iron

limitation, it appears that the conformation of the

complexes themselves and of the membrane as a

whole is impaired. As a result, membranes

become less flexible and the necessary changes

during xanthophyll cycling may be hampered.

The almost full recovery of Fv/Fm under both

iron conditions indicated successful photoaccli-

mation. Damage to the photosystems can, how-

ever, not be excluded. Minor degradations will go

by unnoticed when the rate of repair is sufficiently

fast to sustain high rates of photochemistry

(Morales et al. 1998; Han et al. 2000; Kana et al.

2002; van Leeuwe et al. 2005). The increased

xanthophyll pool of iron-limited cells indicates

that these cells experience an enhanced risk of

photodamage, which could result in increased

energy costs for repair. This exerts additional

stress on iron-limited cells. Whereas iron-suffi-

cient cells invest in larger photosystems (in-

creased Chl a per cell) in order to efficiently use

the periods of low irradiance, iron-limited cells

have to invest in a mechanism to prevent photo-

damage and at the same time maximise energy

production. This is attained by a relative reduc-

tion of (iron-expensive) Chl a per cell, which

results in a reduced packaging effect (Greene

et al. 1991; van Leeuwe and De Baar 2000) and a

relative increase in light-harvesting pigments

(increased ratio of total carotenoid pigments to

Chl a). As a result, the Chl a-specific absorption

cross section �aph significantly increases under iron

limitation. As discussed, the single-cell form that

dominated under iron-deplete conditions may

further contribute to an increase in �aph. The

increase in absorption capacity appears to bal-

ance the reduction in photosynthetic efficiency to

a large extent. If we consider growth as a function

of these photosynthetic parameters we may esti-

mate the relative carbon-normalised growth

capacity (Table 2: rgc) by applying a simple

relationship: rgc � Fv/Fm*�aph*Chl a cell–1/POC

cell–1. This exercise shows that the capacity of

iron-limited and iron-replete cells is almost equal:

0.13 and 0.12, respectively. It should be noted that

this only is an approach of gross growth capacity,

since maintenance costs are not included in this

concept. Net growth rates as determined on the

basis of cell divisions are significantly higher in

iron-replete cells.

The ratio HEXA/Chl a was found to be twice

as high in iron-limited cells compared to iron-

replete cells. In previous experiments, similar

high HEXA/Chl a ratios were recorded (van

Leeuwe and Stefels 1998). Concomitant high

ratios of HEXA/FUCO under high irradiance

gave rise to a hypothesis on an alternative

photoprotective pigment cycle. It was suggested

that in response to exposure to high irradiance the

efficient light-harvesting pigment FUCO would

be converted into its structural relative HEXA.

The efficiency of energy transfer by HEXA has

not yet been described, which leaves room for a

role as energy quencher. This hypothesis could

not be confirmed in our short-term experiments.

The ratio of HEXA/FUCO was again higher for

68 Biogeochemistry (2007) 83:61–70

123

iron-limited cells, but did not vary with fluctuating

irradiance levels. As for the well-established

xanthophyll pigment cycle, such a relationship

should have been apparent in case of an active

role in photochemical quenching. The role of

HEXA is still enigmatic, but an influence on

energy dissipation cannot be excluded.

This study has shown that Phaeocystis antarc-

tica can readily respond to fast changes in light

climate. Iron limitation exerted stress on the

cells, but due to changes in pigment composition

these cells were able to acclimate and apparently

did not suffer from photoinhibition. Altogether,

these characteristics are vital to maintain growth

in the Antarctic Ocean, where deep wind-mixed

layers predominate and where low iron concen-

trations suppress algal growth. Under such con-

ditions P. antarctica appears a successful

competitor. The alga may support carbon flow

within so-called retention systems. Alternatively,

Phaeocystis can contribute to large carbon fluxes

from the surface waters into the oceans interior.

So far, very few data are available on photosyn-

thetic responses towards variable light condi-

tions. With this paper we hope to improve our

understanding of biogeochemical cycles in the

Antarctic ecosystem.

Acknowledgements We would like to thank RonaldVisser for pigment analysis and technical support. Wethank Walker Smith for a critical review of the manuscript.MAvL and JS were financially supported by the DutchPolar Programme of the Research Council for Earth andLife Sciences (ALW) of the Netherlands Organisation forScientific Research (NWO).

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